Genetic diseases are caused by abnormalities in genes or chromosomes. Such abnormalities may include insertions, deletions, and expansions. Huntington's Disease (HD) is one example of a genetic disease caused by an expansion. HD is a progressive neurodegenerative disorder that is inherited in a dominant fashion and results from a mutation that expands the polymorphic trinucleotide (CAG) tract in the huntingtin gene (HTT). The average CAG tract size in the general population is 17-26 repeats (wild type allele), however, in HD patients the CAG tract has expanded to 36 repeats or more (mutant allele) (Huntington's Disease Collaborative Research Group 1993. Cell 72(6):971-83). The HTT gene encodes the HTT protein and the expanded CAG tract results in a pathological increase in the polyglutamine repeats near the N-terminal of the protein. Individuals carry two copies of the HTT gene and one mutant allele is sufficient to result in HD.
HTT protein appears to have a role during development of the nervous system and a protective role in cells. In mouse models, constitutive knockout of the HTT gene is lethal during embryonic development (Nasir et al 1995. Cell 81(5):811-23), while adult inactivation of the HTT gene leads to progressive cell death in the brain and the testes (Dragatsis et al 2000. Nat. Genet. 26:300-306). Reduction of huntingtin expression from the wild type allele may, therefore, have negative consequences.
Like HD, there are disorders for which a strategy of selective reduction of a mutant allele would be beneficial. Thus, there remains an unmet need to selectively reduce expression of mutant allelic variants like that of HTT, which are causative of disease, over the wild type variant, which appears to be necessary for normal cellular processes.